1. Field of the Invention
The present invention pertains to an optical element and a light shutter device using the same, and more particularly, to an optical element made of a material having an electro-optical effect, such as PLZT, and a light shutter device using the optical element as a light shutter element.
2. Description of the Related Art
Conventionally, as a light shutter device that forms an image (a latent image) on photographic paper or film using a silver photosensitive material or a photosensitive drum for electronic photography, a device that uses a material having an electro-optical effect, such as PLZT, has been known. Such a light shutter device includes at lease one light shutter element. A light shutter element has a construction in which drive electrodes (a common electrode and an individual electrode) are placed on a chip comprising PLZT, and the area sandwiched by these electrodes is deemed a light modulation region equivalent to one pixel.
Incidentally, the method by which to resolve hysterisis is an important issue in a light shutter device of this type. Hysterisis is the reduction in contrast as the light shutter element is used over time. This phenomenon is presumed to occur due for the following reason. In an electro-optical material, a minute amount of electric charge occurs in areas exposed to light in an environment where an electric field is present. This electric charge accumulates as the exposure to light is repeated. As a result of the accumulation of electric charge, a space electric field is formed inside the light shutter element. This space electric field tends to exist near the electrodes and changes the optimal drive voltage (half-wavelength voltage). It therefore reduces the amount of permeating light while voltage application is ON. The space electric field also increases the amount of leaking light while voltage application is OFF. Consequently, the image contrast decreases as time passes.
To combat this phenomenon, it is conventionally known that light modulation regions should be partially processed (reduced) to change their properties such that their electrical resistance will be reduced (see U.S. Pat. No. 5,745,280, for example). However, this is problematic in that changing the properties of the element itself involves a relatively complex process and that it is difficult to control the level of resistance.